In the metal joints used in such devices, the elastic metal core is constituted either by a metal tube, or by a helical spring with continguous turns and closed on itself. In the latter case, the cross-section of the wire forming the spring can have numerous different shapes and in particular can be circular, rectangular or in circular segment form. The helical spring or tube, which is fastened on the edges, gives the joint its elasticity.
The envelope or envelopes surrounding the core of said joints are made from plastic materials or thin sheets from materials such as polytetrafluoroethylene, aluminium, silver, copper, nickel, tantalum, stainless steel, zirconium, etc.
When the flexible metal joints have several envelopes, each of the latter has its own particular function. Thus, when the core of the joint is constituted by a spring having contiguous turns, the inner envelope generally has a low distribution function at each turn apex of the elastic core. The outer envelope is made from a ductile metal so that, as a result of its creep characteristics in the roughnesses of the contacting surfaces, it gives the joint a perfect seal when it is secured in an assembly. In general terms, the plastic properties of the materials forming the same characterize the envelopes of the joint.
It is also pointed out that existing metal joints generally have a toroidal shape, whilst the envelopes surrounding the elastic core or web are also toroidal and the generator circle is not closed on itself. It can be a torus, if the joint is circular. However, the joint can have any other shape and in particular can be rectangular, oval, square, etc.
An illustration of an existing metal joint, whose elastic core is constituted by a helical spring with contiguous turns, is provided by FR-A-2,232,235.
These joints are generally used in assemblies constituted by two flat flanges or which have a groove with a rectangular section serving to receive the joint. Under these conditions, the behaviour of existing metal joints is perfectly satisfactory.
However, as is more particularly illustrated by FR-A-1,583,060, there are certain applications, particularly petrochemical applications, where the joints also must be placed in grooves having a trapezoidal section and formed in facing flanges. In this case, the joints must ensure both the tightness between the flanges and the relative centering between the latter. Moreover, the operating conditions are generally very difficult, particularly with regards to the very high temperatures and pressures.
The use within such assemblies of existing flexible metal joints with a circular section is not particularly satisfactory, particularly due to the relatively great depth of the grooves as compared with their width.
Furthermore, U.S. Pat. No. 1,985,475 discloses a flexible metal joint with an oval cross-section, which is more particularly designed for withstanding the high pressure prevailing in internal combustion engines.
However, it is not easy to use such a joint in an assembly of the type described in FR-A-1,583,060. Thus, if certain conditions revealed by the said specification are not respected, the fastening of the assembly leads to sliding and flattening of the envelopes surrounding the core or web of the joint. As a result, the oval of the joint is deformed, so that the joint is weakened and cannot be reused.